1. Field of the Invention
The present invention generally relates to the field of semiconductor processing. More particularly, the present invention relates to methods and apparatus for thermally processing a semiconductor substrate.
2. Description of the Related Art
During semiconductor processing, substrates may be heated to high temperatures so that various chemical and/or physical reactions can take place. Thermal processes are usually used to heat the substrates. A typical thermal process, such as annealing, requires providing a relatively large amount of thermal energy to the substrate in a short amount of time, and thereafter rapidly cooling the wafer to terminate the thermal process. Examples of thermal processes currently in use include Rapid Thermal Processing (RTP) and impulse (spike) annealing.
In general, these thermal processes heat the substrates under controlled conditions according to a predetermined thermal recipe. These thermal recipes fundamentally consist of a temperature that the semiconductor substrate must be heated to the rate of change of temperature, i.e., the temperature ramp-up and ramp-down rates and the time that the thermal processing system remains at a particular temperature.
Accurate temperature measurement is essential to control the process to achieve desired result and to maintain uniformity across the substrate during thermal processing. Temperature measurement of substrates being processed or chamber components is usually conducted in-situ by non-contact methods, such as using pyrometers to sense radiation energy from the target objects. A pyrometer generally detects energy level of a certain wavelength of radiant energy from the target objects to determine temperature of the target objects.
Accuracy of temperature measurement by pyrometers during thermal processing is usually affected by noises received in the radiant energy. For example, when measuring a temperature of a substrate being processed, radiant energy from the energy source may be directly or through reflection received by the sensor in addition to the radiant energy emitted by the substrate.
Various methods have been used to prevent unwanted radiant energy from entering a temperature sensor. For example, covers and shields are used to prevent background noises, such as radiation from the energy source, from entering the sensor. However, the covers and shields not only increase system complexity, reduce system flexibility, but also limit the lower range of the sensors.
Spectral filters are also used to filter out background radiant energy within a working wavelength of the radiant sensor. The working wavelength is generally within a limited range for measuring a semiconductor substrate during thermal processing. Suitable spectral filters, such as a spectral filter containing rare earth elements, are usually expensive. Additionally, spectral filters usually absorb radiant energy from the heating source within the working wavelength. The absorption not only amounts waste of the radiant energy from the heating source, but also causes overheat of the spectral filters themselves.
Therefore, there is a need for improved apparatus and methods for efficient and accurate temperature measurement during thermal processing.